httppeer: remove redundant code to fetch capabilities
_fetchcaps() is called by httppeer.instance(), which is the only
instantiator of httppeer. Since _fetchcaps() always sets self._caps
and since 197d10e157ce removed the fallback for cases where the
remote doesn't support capabilities, we can remove some dead
code from httppeer.capabilities().
Differential Revision: https://phab.mercurial-scm.org/D2215